18 BULLETIN 829, U. S. DEPARTMENT OF AGRICULTURE. 



by some principle present only in dieased plants is responsible for the 

 appearance of the disease in formerly healthy individuals. 



TRANSMISSION OF MOSAIC IN DISEASED SEED PIECES. 



Experiments in Porto Rico 1 - and elsewhere have repeatedly demon- 

 strated that cuttings from infected stalks invariably give rise to 

 infected jjlants. The young shoots are seen to be mottled as soon as 

 they appear. These are referred to as primary infections. The 

 fact is one of far-reaching importance, and to it must be attributed the 

 spread of the disease to new regions, remote from any infected cane, 

 by shipments of cane seed. The use of diseased stalks for propagating 

 results in wider distribution of diseased plants on the same planta- 

 tion from year to year and insures the survival of the virus, even in 

 the absence of secondary infections. Transmission of the disease in 

 cuttings is a fact, the importance of which can not be overemphasized 

 in view of its obvious bearing on control measures. 



TRANSMISSION OF THE DISEASE BY CARRD3RS. 



It can be proved mathematically that by the law of chance the 

 percentage of diseased plants in a plantation would tend to remain 

 stationary from year to year provided there was no conscious or 

 unconscious selection, 2 if the spread of the disease depended wholly 

 upon the use of infected cuttings. Nature has provided a far more 

 efficient method for the quick dissemination of the malady. Second- 

 ary infection, i. e., infection due to the inoculation of healthy plants 

 during the growing season, goes on at a more or less rapid rate wher- 

 ever the disease has been observed. Secondary infections are easily 

 determined as such when the plants are young. In the case of plants 

 infected in the greenhouse it has been determined that only the leaves 

 which were immature at the time of inoculation and leaves subse- 

 quently formed become mottled. When a plant is found with normal 

 leaves up to a certain point on the stalk and mottled leaves above 

 that point it is a clear case of secondary infection. Since in older 

 plants the lower leaves are gradually sloughed off until only a 

 relatively small terminal tuft of the youngest leaves remain when the 

 plant approaches maturity, this method is obviously limited to 

 young plants or to plants with green leaves still present above and 

 below the point of inoculation. 



The rate of spread of the disease, as indicated by these secondary 

 infections, varies greatly. Fields are frequently seen in which there 

 has been apparently no secondary infection during an entire growing 



1 Stevenson, John A. The "mottling" disease of cane. Porto Rico Insular Exp. Sta. Ann. Rpt. 1916-17, 

 p. 40-77. 1917. [Literature], p. 76-77. 



2 Selection is employed where the disease is not recognized. During the beginning of the epidemic in 

 Porto Rico, when sugar was bringing an unprecendented price, it was learned that the manager of one of the 

 mills was instructed to grind the best cane and save the poorest for seed. The "poorest" was undoubtedly 

 that attacked bv mosiac. 



